High-current current-limiting fuse

ABSTRACT

A high-current current-limiting fuse comprises an aluminum plate containing two spaced-apart notches extending transversely of the central longitudinal axis of the plate from opposite lateral edges of the plate to points near said axis, thereby dividing the plate into two massive tang portions spaced apart along said axis and a restricted neck portion interconnecting the tang portions. The tang portions constitute fuse terminals, and the neck portion constitutes fusible conducting means for carrying current between the tang portions and for melting in response to overcurrents. The neck portion is integral with the tang portions, and the junctions between the neck portions and the tang portions are free of solder. A housing of insulating material surrounds the neck portion and comprises plates of insulating material at opposite faces of the aluminum plate and portions of insulating material extending into the notches. The insulating plates are secured to said tang portions and to each other, thereby effectively preventing motion of the tang portions relative to each other.

United States Patent [1 1 Bogert et al.

[451 May 13,1975

[ HIGH-CURRENT CURRENT-LIMITING- FUSE [75] Inventors: Ernest H. Bogert,Media; Peter Kotos, Havertown, both of Pa.

[73] Assignee: General Electric Company,

1 Philadelphia, Pa.

[22] Filed: Apr. 1, 1974 [21] Appl. No.: 456,701

[52] U.S. Cl. 337/295; 337/159; 337/290 [51] Int. Cl. H0lh 85/04 [58]Field of Search 337/290, 292, 295, 296,

[56] References Cited UNITED STATES PATENTS 2,734,112 2/1956 Kozacka337/159 X 2,781,434 2/1957 Swain 337/159 2,861,150 11/1958 Swain 337/1593,288,968 11/1966 Feenan et al. 337/295 Primary Examiner-J. D. Miller IAssistant Examiner-Fred E. Bell Attorney, Agent, or Firm-J. WesleyHaubner; William Freedman [57] ABSTRACT A high-current current-limitingfuse comprises an aluminum plate containing two spaced-apart notchesextending transversely of the central longitudinal axis of the platefrom opposite lateral edges of the plate to points near said axis,thereby dividing the plate into two massive tang portions spaced apartalong said axis and a restricted neck portion interconnecting the tangportions. The tang portions constitute fuse terminals, and the neckportion constitutes fusible conducting means for carrying currentbetween the tang portions and for melting in response to overcurrents.The neck portion is integral with the tang portions, and the junctionsbetween the neck portions and the tang portions are free of solder. Ahousing of insulating material surrounds the neck portion and comprisesplates of insulating material at opposite faces of the aluminum plateand portions of insulating material extending into the notches. Theinsulating plates are secured to said tang portions and to each other,thereby effectively preventing motion of the tang portions relative toeach other.

10 Claims, 5 Drawing Figures v I, 48 '42- i i \42 az F, o l i;

\\J Jlj t if 42@|" v I 48- 37 {6F HIGH-CURRENT CURRENT-LIMITING FUSEBACKGROUND This invention relates to a high-current, currentlimitingfuse that is characterized by exceptional simplicity, exceptionally lowheat losses, and exceptional ability to withstand high temperatureswithout changing its operating characteristics.

The type of fuse that we are concerned with is required to carry currentin the kiloampere range on a steady-state basis and to interrupt withcurrent-limiting action circuits where the available current is in therange of hundreds of kiloamperes. The usual fuse that is intended forsuch duty comprises many separate, very thin fusible elements and twomassive end plates of conductive material between which the fusibleelements extend electrically in parallel. It is not uncom-' mom for sucha fuse to have 50 or more such fusible elements in parallel. Thesefusible elements are typically joined to the conductive end plates bysoldering. The end plates usually are provided with conductive tangsserving as terminals for the fuse that can be bolted to buses or othercircuit components.

A problem sometimes present with such a fuse is that the relatively hightemperatures developed by high continuous currents passing through thebolted terminal connections and through the end plates, the fusibleelements, and the joints therebetween can cause undesirable melting ofthe solder in the joints, thus forming electric arcs between the fusibleelements and the end plates. This is an undesirable location for arcingbecause the arc-extinguishing medium of the fuse (usually quartz sand)will not be in close contact with such arcs, and they will burn holesinto the end plates and expel large amounts of molten metal and vaporinto the surrounding region, thus creating an unduly high probabilitythat the fuse will fail to interrupt the current.

Even a higher-than-normal temperature that is well below the meltingpoint of the solder can produce an alloying action between the solderand the metal of the fusible element that lowers the melting point ofthe fusible element at the joint and increases its resistance at thejoint, resulting in undesirable changes in the current-time operatingcharacteristics of the fuse as well as the undesirable arcing conditionnoted in the preceding paragraph.

SUMMARY ing material or some other attachment, such as the M- effectsolder bead. This can be quite expensive when it is considered thatthere often are 50 or more fusible elements requiring such treatment,and all of these elements must be individually attached to the endplates. Further complicating matters is that in such prior fuses,

the fusible elements are usually relatively thin and fragile, and thisnecessitates special care in their handling.

The parallel-connected fusible elements in the typical prior fuse areembedded in a suitable filler, such as quartz sand; and the tiller ishoused within a large insulating casing. Filling the casing with sandthat is tightly packed about the fragile fusible elements requiresspecial care to avoid damage to the fusible elements.

Another object of our invention is to construct the fuse in such amanner that the fusible elements can be drastically reduced in number toonly one, and, in particular, to one which is much more rugged than thefragile elements referred to hereinabove.

Still another object is to drastically reduce the quantity of fillerneeded in comparison to that used in the above-described prior fuses andto reduce the requirements for care in filling the fuse casing.

Still another object is to construct the fusible element in such amanner that it has an exceptionally large conductive mass immediatelyadjacent its reduced crosssection portion with very low thermalresistance intervening between the mass and the reduced cross-sectionportion. This enables the mass to act as an effective heat-sink coolerfor the reduced cross-section portion, thereby permitting highersteady-state currents without overheating.

In carrying out the invention in one form, we provide a plate of highconductivity metal having a central longitudinal axis and twospaced-apart notches extending transversely of the axis from oppositelateral edges of the plate to points near the axis. The notches dividethe plate into two tang portions spaced apart along said longitudinalaxis at opposite sides of the notches and a neck portion interconnectingthe tang portions and located between the inner ends of the notches. Thetang portions serve as terminals of the fuse, and the neck portionserves as fusible conducting means for carrying current between the tangportions and for melting in response to overcurrents. The neck portionis integral with the tang portions, and the junctions between the neckportion and the tang portions are free of solder. The neck portion, thetang portions, and the junctions are all of the same material and formedfrom the same part. A housing of insulating material surrounds the neckportion and comprises plates of insulating material at opposite faces ofthe conductive plate and portions of insulating material extending intothe notches. The insulating plates are secured to the tang portions andto each other, thereby effectively preventing motion of the tangportions relative to each other. The tang portions have end regionslocated outside said housing that are adapted to be secured to externalcircuitry.

BRIEF DESCRIPTION OF DRAWINGS For a better understanding of theinvention, reference may be had to the following description taken inconjunction with the accompanying drawings, wherein:

FIG. 1 is a plan view of a current limiting fuse embodying one form ofthe invention.

FIG. 2 is an exploded perspective view of certain major components ofthe fuse of FIG. 1.

FIG. 3 is a side view, partly in section, of the fuse of FIG. 1.

FIG. 4 is a sectional view along the line 4-4 of FIG. 1.

FIG. 5 is aside view of another modified embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIGS. 1and 2, the illustrated fuse comprises a unitary plate of highlyconductive material, preferably aluminum, from which all of theconductive parts of the fuse are formed. This plate 10 has a centrallongitudinal axis 11 and flat upper and lower faces 12 and 13 at itsopposite sides. Plate 10 contains two spaced-apart notches 16 extendingtransversely of its longitudinal axis 11 from opposite lateral edges ofplate 10 to points near the axis. These notches 16 are located inaligned positions along the axis 11. Notches l6 divide the plate 10 intotwo tang portions 20 and 21 spaced apart along the longitudinal axis 11and located at opposite sides of the 'notches. Between the inner ends ofthe notches there is defined a neck portion 24 that interconnects thetang portions 20 and 21.

As will soon appear more clearly, the tang portions 20 and 21 constitutefuse terminals, and the neck portion 24 constitutes fusible conductingmeans for carrying current between the tang portions and for melting inresponse to overcurrents. The neck portion is integral with the tangportions, being machined or otherwise formed out of the same unitaryplate as the tang portions are formed. In view of this integralconstruction, the junction between the neck portion and each of the tangportions is free of solder and brazing material and is of the samematerial as the neck portion and the tang portions.

The notches 16 have V-shaped inner ends and, as a result, the neckportion 24 has V-shaped edges and a region of minimum cross section nearits center substantially equidistant from the tang portions. As will beapparent from FIGS. 2 and 4, the neck portion 24 is spaced from theupper face 12 of plate 10. Note further that each tang portion containsa recess 32 in its edge facing the other tang portion, and this recessextends from the upper face 12 to the location of the neck portion. Aswill be apparent from FIG. 2, these recesses 32 are aligned withthe neckportion and thus provide at the end of each recess a reduced thicknessregion 34 of the tang portion forming an extension of the neck portion24. The significance of the reduced thickness region 34 will soon appearmore clearly.

Surrounding the neck portion 24 is a housing 35 of insulating material.This housing 35 comprises two thick plates 37 and 38 of insulatingmaterial, for example, a melamine resin. These insulating plates 37 and38 are located at opposite faces of the conductive plate 10, and one ofthe plates has a pair of ribs 40 projecting toward the other plate andfitting snugly in the notches 16.

The insulating plates 37 and 38 are clamped to opposite faces of theconductive plate 10 and to each other by means of bolts 42 extendingthrough registering holes in the plates and nuts 43 threaded on thebolts. Suitable lockwashers are provided between the nuts 43 and one ofthe insulating plates to maintain the desired clamping pressure despitecreepage of the insulating material of the plates 37 38.

As shown in FIG. 2, each of the insulating plates is hollowed out at 46in the region of the neck portion 24 so that the inner wall of theinsulating housing is spaced from the neck portion at all points on theneck portion.

As shown in FIG. 4, the space between housing 35 and neck portion 24 isfilled with a granular arcextinguishing material 45, such as quartzsand. A suitable hole 47 is provided in the housing wall topermitintroduction of the filler. The filler material can be tampedthrough this hole to pack it tightly about the neck portion 24, afterwhich the hole is suitably plugged, as by a screw 49. It will be notedthat the filler completely fills the recesses 32 and is therefore inintimate contact with the reduced thickness portions 34 at the ends ofthe neck portion.

In the form of the invention in FIGS. 1-4, a sealant is provided alongthe entire interface between the housing 35 and plate 10 to seal thecavity within the housing from the exterior, thereby preventing anyfiller from escaping and also confining within the cavity the arcingproducts formed upon fuse operation. A suitable sealant is aroom-temperature-vulcanizing silicone cement.

The fuse of FIGS. 1-4 is incorporated in the circuit that it is intendedto protect by bolting its tangs 20 and 21 to the adjacent circuitelements. Suitable holes 48 are provided in the tangs to accommodate thebolts that are used for this purpose. As previously noted, current flowsbetween the tangs 20 and 21 via neck portion 24. The neck portionremains cool enough to carry rated steady-state current indefinitelywithout melting. But if the current should abruptly rise, say inresponse to a short circuit condition, the neck portion will rapidlymelt, thus forming an are within housing 35. The surrounding filler 45reacts with the arc in a known manner to develop a high are voltage thatlimits the current let through by the fuse to a value substantiallybelow the peak of the available current.

The fuse of FIGS. 14 is capable of carrying steadystate currents in thekiloampere range and of interrupting with current-limiting actionavailable currents in the hundreds of kiloampere range. In one specificembodiment, the plate 10 is of aluminum having 56 percent of theconductivity of copper and is approximately inch thick, 3% inch wide,and 3 l5/l6 inches long. The notches 16 are approximately 0.312 inch inwidth with their inner ends spaced by about 0.145 inch. Each edge of theneck 24 forms A V-angle of about 50 degrees, and the neck is about inchthick, as measured vertically in FIG. 4. Such a fuse can carry on asteadystate basis a current of 2650 amperes rms and can limit to 100,000amperes the peak let-through current in a volt rrns 60 Hz circuit with150,000 amperes rms symmetrical available current. These figures areprovided by way of example and not limitation.

The high steady-state current-carrying capability of the fuse is madepossible by the fact that the massive tang portions 20 and 21 are invery close proximity to the neck portion 24 and are integral with theneck portion, with no solder being present at the junctions between theneck portion and the tang portions. This integral construction providesexceptionally low electrical resistance to limit 1 R heating at thejunction and also exceptionally low thermal resistance to heat generatedat the neck portion and flowing into the tang portions. The absence ofsolder in the junction regions desirably eliminates any alloying actionthat could result from exposing the solder to high temperatures. Suchalloying typically increases the resistance of the affected metal,resulting in further heating. 3

Another source of heat that can present problems is the bolted joint(not shown) at each fuse terminal. I-Iigh currents passing through thisbolted joint will develop heat that contributes to heating of theconductive plate 10. If any solder were present in the neck-to-ta'ngjunctions, the temperature of the solder could be elevated sufficientlyby such heating to produce alloying and the resultant undesirableincrease in electrical resistance at the junctions. By eliminating anysolder in these junction regions, we are able to avoid such alloying inthe junctions.

The highly effective cooling of the neck portion 24 made possible by thelarge conductive masses immediately thereadjacent and the low thermalresistance at the neck-to-tang junctions make it possible for us toutilize only a single neck portion for the high continuous currentsinstead of the many elements in parallel previously used. This greatlysimplifies the fuse and eliminates the manufacturing complicationsresulting from forming, treating, and handling many fuse ele-' ments ina single fuse.

While it is possible to reduce heating under continuous currentconditions by making the neck portion 24 large in cross section, it isto be understood that the maximum cross-section is limited by the needfor the neck portion to melt in the required times under shortcircuitconditions.

Effective current-limiting action under short circuit conditions dependsupon the arc being confined and not allowed to spread over the width ofthe fuse element. The housing 35 which closely surrounds the neckportion 24 contributes to such confinement. The intimate contact that ispresent between the arc and the surrounding quartz sand also plays animportant role in the current-limiting interrupting process by intenselycooling the arc. By providing the recesses 32 in the edges of the tangportions and 21, we are able to maintain the quartz sand in contact withthe arc along its entire length even after its terminals have burnedback into the tang portions. In this regard, note that the quartz sandis in intimate contact with the thin regions 34; and even if the arcterminals burn into these thin regions, there is sand immediatelyadjacent them to react with the arc.

It is to be noted that our fuse of FIGS. 14 uses only a very smallamount of arc-extinguishing filler material.

Since the fuse comprises only a single fusible element,

arcing upon fuse operation is confined to a very restricted region andonly a small quantity of filler is needed to provide a surrounding bodyof sand. Where multiple parallel fuse links have been present, it hasbeen customary to provide a single large body of filler in which all thelinks are embedded.

Using a single fusible element 24 instead of many in parallel means thatthe fusible element will have a much larger cross-section than each ofthe parallelconnected elements and will therefore be much less fragile.This enables filler to be introduced into the housing and packed tightlyby vibration and tamping with less concern about damaging the fuseelement as a result of the associated forces.

In addition to confining the arc, housing 35 serves also to protect theneck portion 24 against possible physical damage as a result of roughhandling of the fuse. Since the thick insulating plates 37 and 38 aretightly clamped to the rigid tang portions 20, 21 and to To provide anindication of when the fuse has operated, we provide exteriorly of thefuse housing 35 a high resistance indicator wire 55 that extends betweentwo terminals 58 and 59 respectively secured to the two tangs 20 and 21.When the fuse operates, current is shunted through this high resistancewire 55 to melt it and indicate fuse operation, all in a conventionalmanner. By locating the indicator wire terminals 58, 59 onlaterally-opposed sides of the housing 35 instead of on the same side,they are a greater distance apart and there is less likelihood of anelectricalbreakdown between them upon melting of the indicator wire. Toprotect the indicator wire against physical damage prior to its melting,a pair of notches 57 (FIG. 4) are provided in the exposed corners of thehousing 35 to receive and shield the wire. The notches 57 serve also toposition the indicator wire in a position midway between the heads ofbolts 42, thus reducing the likelihood of contact between the wire andboltheads.

Referring to FIG. 1, it will be noted that the tangs project laterallybeyond the housing 35, providing exposed portions adjacent thelaterally-opposed edges of conductive plate 10 to which the terminals 58and 59 can be attached. These exposed portions also serve as fins forimproving heat transfer to the surrounding ambient as well as providingadditional cross-section for reducing the electrical resistance arid;resultant heating of the tangs.

For much higher current applications than those referred to hereinabove,the fuse assembly of FIG. 5 can be used. This assembly comprises twofuses in parallel, each of the same construction as the fuse of FIGS.l-4 except that one of the insulating plates 38 serves as a portion ofthe housing 35 of both fuses. Bolts 42 extending through the threeinsulating plates 37, 38, 38a and the conductive plates 10 clamp theseparts together in fixed relationship to each other. The centralinsulating plate 38 is hollowed out on both its upper and lower faces toform a part of the cavity surrounding the neck portion of each fuse.

While we have shown and described particular embodiments of ourinvention, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from ourinvention in its broader aspects; and we, therefore, intend in theappended claims to cover all such changes and modifications as fallwithin the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A current-limiting fuse comprising:

a. a plate of high-conductivity metal having a central longitudinal axisand two spaced-apart notches extending transversely of said axis fromopposite lateral edges of said plate to points near said axis, therebydividing said plate into: (a,) two tang portions spaced apart along saidlongitudinal axis at opposite sides of said notches and (a a neckportion interconnecting said tang portions and located between the innerends of said notches,

b. said tang portions constituting fuse terminals and said neck portionconstituting fusible conducting means for carrying current between saidtang portions and for melting in response to overcurrents,

c. said neck portion being integral with said tang portions, thejunctions between said neck portion and said tang portions being free ofsolder, and the neck portion, the tang portions, and said junctions allbeing of the same material and formed from the same part,

(1. each of said tang portions having a relatively large cross-sectionin any plane therethrough normal to said longitudinal axis compared tothe crosssection of said neck portion in any plane therethrough normalto said longitudinal axis,

e. a housing of insulating material surrounding said neck portionandcomprising: plates of insulating material at opposite faces of saidconductive plate and portions of insulating material extending into saidnotches,

f. and means for securing said insulating plates to said tang portionsand to each other, thereby effectively preventing motion of said tangportions relative to each other,

g. said tang portions having'end regions located outside said housingadapted to be secured to external circuitry.

2. The current-limiting fuse of claim 1 in which said insulating housinghas an inner wall that surrounds said neck portion and is spacedtherefrom, and which further comprises: a filler of granulararc-extinguishing material filling the space between said neck portionand said inner wall.

3. The current-limiting fuse of claim 2 in which sealing means isprovided between said housing and said conductive plate to confine thefiller to the space within said housing and to confine arcing productsto said space.

4. The current-limiting fuse of claim 1 in which said integral tangportions and said neck portion are of aluminum.

S. The current-limiting fuse of claim 1 in which said neck portion isspaced from at least one face of said plate; and each of said tangportions, at its edge facing the other tang portion, has a recesstherein extending from said one face to the location of said neckportion; said recess being aligned with said neck portion, therebyproviding at the end of said recess a reduced thickness region of saidtang portion forming an extension of said neck portion.

6. The current-limiting fuse of claim 5 in which said insulating housinghas an inner wall that surrounds said neck portion and is spacedtherefrom, and which further comprises: a filler of granulararc-extinguishing material filling the space between said neck portionand said inner wall; said filler filling said recesses in the tangportions thereby provide filler in contact with said reduced thicknessregions forming extensions of said neck portion.

7. The current-limiting fuse of claim 1 in which said plate is severalinches in width as measured between its lateral edge and said neckportion has a minimum crosssection zone less than about /4 inch asmeasured in the same direction.

8. The current-limiting fuse of claim 1 in which said tangs projectlaterally outward beyond the boundaries of said insulating housing, thusforming exposed fins adjacent said notches for facilitating cooling ofsaid conductive plate. A

9. The current-limiting fuse of claim 1 in combination with anindicating wire electrically in parallel with said neck portion, locatedexternally of said housing, and connected between two terminal points onsaid spaced tang portions located adjacent the laterallyopposed edges ofsaid conductive plate.

10. The current limiting fuse of claim 1 in combination with anindicating wire electrically in parallel with said neck portion, locatedexternally of said housing, and connected between two terminal points onsaid exposed fins at laterally-opposed sides of said housing.

1. A current-limiting fuse comprising: a. a plate of high-conductivitymetal having a central longitudinal axis and two spaced-apart notchesextending transversely of said axis from opposite lateral edges of saidplate to points near said axis, thereby dividing said plate into: (a1)two tang portions spaced apart along said longitudinal axis at oppositesides of said notches and (a2) a neck portion interconnecting said tangportions and located between the inner ends of said notches, b. saidtang portions constituting fuse terminals and said neck portionconstituting fusible conducting means for carrying current between saidtang portions and for melting in response to overcurrents, c. said neckportion being integral with said tang portions, the junctions betweensaid neck portion and said tang portions being free of solder, and theneck portion, the tang portions, and said junctions all being of thesame material and formed from the same part, d. each of said tangportions having a relatively large crosssection in any planetherethrough normal to said longitudinal axis compared to thecross-section of said neck portion in any plane therethrough normal tosaid longitudinal axis, e. a housing of insulating material surroundingsaid neck portion and comprising: plates of insulating material atopposite faces of said conductive plate and portions of insulatingmaterial extending into said notches, f. and means for securing saidinsulating plates to said tang portions and to each other, therebyeffectively preventing motion of said tang portions relative to eachother, g. said tang portions having end regions located outside saidhousing adapted to be secured to external circuitry.
 2. Thecurrent-limiting fuse of claim 1 in which said insulating housing has aninner wall that surrounds said neck portion and is spaced therefrom, andwhich further comprises: a filler of granular arc-extinguishing materialfilling the space between said neck portion and said inner wall.
 3. Thecurrent-limiting fuse of claim 2 in which sealing means is providedbetween said housing and said conductive plate to confine the filler tothe space within said housing and to confine arcing products to saidspace.
 4. The current-limiting fuse of claim 1 in which said integraltang portions and said neck portion are of aluminum.
 5. Thecurrent-limiting fuse of claim 1 in which said neck portion is spacedfrom at least one face of said plate; and each of said tang portions, atits edge facing the other tang portion, has a recess therein extendingfrom said one face to the location of said neck portion; said recessbeing aligned with said neck portion, thereby providing at the end ofsaid recess a reduced thickness region of said tang portion forming anextension of said neck portion.
 6. The current-limiting fuse of claim 5in which said insulating housing has an inner wall that surrounds saidneck portion and is spaced therefrom, and which further comprises: afiller of granular arc-extinguishing material filling the space betweensaid neck portion and said inner wall; said filler filling said recessesin the tang portions thereby provide filler in contact with said reducedthickness regions forming extensions of said neck portion.
 7. Thecurrent-limiting fuse of claim 1 in which said plate is several inchesin width as measured between its lateral edge and said neck portion hasa minimum crosssection zone less than about 1/4 inch as measured in thesame direction.
 8. The current-limiting fuse of claim 1 in which saidtangs project laterally outward beyond the boundaries of said insulatinghousing, thus forming exposed fins adjacent said notches forfacilitating cooling of said conductive plate.
 9. The current-limitingfuse of claim 1 in combination with an indicating wire electrically inparallel with said neck portion, located externally of said housing, andconnected between two terminal points on said spaced tang portionslocated adjacent the laterally-opposed edges of said conductive plate.10. The current limiting fuse of claim 1 in combination with anindicating wire electrically in parallel with said neck portion, locatedexternally of said housing, and connected between two terminal points onsaid exposed fins at laterally-opposed sides of said housing.